There is a continual need to move items from one location inside of a factory or warehouse to another. This often involves moving the items up or down steep inclines, as well as horizontally. Various systems have been developed by which material can be moved automatically along a selected conveyance path through a factory or warehouse. One such system is described in U.S. Pat. No. 4,438,701 to Murai et al.
In the Murai patent, a system is set forth wherein a series of trolleys are guided along a trolley track by a power conveyor (usually a chain type). The power conveyor runs parallel to and, in this case, directly below the trolley track, however, it is commonly positioned above the trolley track also. The trolleys support a carrier upon which a large variety of loads can be placed for transport to any destination along the track. The system set forth in Murai is equipped with a device which will disengage the drive mechanism from the trolleys in the event that the support carrier collides with another carrier in front of it. While this safety feature is clearly very beneficial, it still leaves potential for a serious accident to occur.
For example, a problem that has been experienced with the Murai system is that the trolleys and the power conveyor have been known to become inadvertently disengaged allowing the trolleys to cruise along the track unbridled. Another problem is that a power conveyor malfunction allows the trolleys to travel at uncontrollable speeds along the track. If either of these situations occurs while the trolleys and load carrier are on level track, the situation is generally not that serious, but clearly undesirable. However, as these carriers often transport items as large as automobiles, a power conveyor malfunction or trolley disengagement on a sloped track produces the potential for disaster.
Accordingly, systems have been developed to retard "run away" trolleys propelled along horizontal paths or down vertical slopes. However, these prior art systems are all characterized by various disadvantages.
One such prior art solution has been to install a third conveyor (separate from the power and free conveyor) which is driven by the carrier as it travels down a vertical decline. This third conveyor is equipped with a speed sensing device which, when an over speed situation is detected, activates a brake. The brake then applies a torque force to the head shaft of the third conveyor which, because it is connected to the carrier, brings the carrier to a stop. A disadvantage to this prior art solution is that it is very expensive because it requires an entirely separate conveyor in addition to the already existing track network. Also, since an additional conveyor is a separate system from the main track network, it requires a substantial amount of additional space. In manufacturing, space is at an absolute premium and any system requiring large amounts of space is considered to have serious drawbacks.
Another solution also involves a separate conveyor which is connected to the carrier. In this solution the separate conveyor is electrically driven at a speed slightly greater than that at which the power conveyor which engages the trolleys is driven. During normal operation the power conveyor acts as a hold back to the force of the separate conveyor. In the event that the trolleys become disengaged from the power conveyor, the carrier will speed up to the pace of the separate conveyor but will go no faster.
Clearly, as this solution requires the installation and operation of an additional conveyor system, the same disadvantages that are experienced with the first noted prior art solution are experienced with this solution as well.
A third prior art solution involves an inertially triggered device that, in the event an over speed situation occurs, throws a stop in front of the carrier. While this system is less expensive than other prior art solutions, it stops the carrier in an extremely abrupt fashion which is not desirable. Moreover, by virtue of the principles upon which this system operates, it is very difficult to cushion the carrier deceleration.
In short, systems for the transportation of heavy, bulky items within a manufacturing plant should be compact, inexpensive and easily installed and include a mechanism by which loaded carriers traveling freely down sloped tracks can be gradually brought to a stop.